1. Field of the Invention
The present invention relates to an optical recording and/or reproducing apparatus, and more particularly, to a compatible optical pickup capable of compatibly adopting a high-density recording medium, which uses a blue light, and a low-density recording medium having a lower recording density than the high-density recording medium, and an optical recording and/or reproducing apparatus adopting the compatible optical pickup.
2. Description of the Related Art
Optical recording and/or reproducing apparatuses record information in or reproduce information from a disc-type recording medium using a light spot focused by an objective lens. In optical recording and/or reproducing apparatuses, a recording capacity depends on a size of the light spot. The size (S) of the light spot is determined depending on a wavelength (λ) of light and a numerical aperture (NA) of an objective lens as expressed in Equation 1:S∝λ/NA  (1)
As shown in Equation 1, the size of the light spot depends on the wavelength of light used and the numerical aperture of the objective lens.
Since CDs on and/or from which information can be recorded and/or reproduced using light with a 780 nm wavelength and an objective lens having a numerical aperture of 0.45 or 0.5 emerged, many lines of research into an improved information storage medium with an increased information storage capacity by increasing a recording density have been made. This research resulted in digital versatile discs (DVDs), on and/or from which information is recorded and/or reproduced using light with a 650 nm wavelength and an objective lens with a numerical aperture of 0.6 or 0.65.
At present, additional research into high-density recording media providing a recording capacity of 20 GB or greater using light with a blue wavelength, e.g., a 405 nm wavelength, have been made steadily.
High-density recording media are actively under standardization, and standardization of some of the high-density recording media is nearly completed. The high-density recording media uses light with a blue wavelength, e.g., a 405 nm wavelength. An objective lens used for the high-density recording media has a numerical aperture of 0.65 or 0.85.
CDs have thicknesses of 1.2 mm, and DVDs have thicknesses of 0.6 mm. The reason why DVDs have thicknesses of 0.6 mm smaller than 1.2 mm is to secure a tolerance caused by the tilt of a recording medium, because DVDs have numerical apertures of about 0.6, which is greater than a 0.45 numerical aperture of CDs.
When a tilt angle of a recording medium is θ, a refractive index of the recording medium is n, a thickness of the recording medium is d, and a numerical aperture of the recording medium is NA, a coma aberration W31 generated by a tilt of the recording medium may be expressed in Equation 2:
                              W          31                =                              d            2                    ⁢                                                                      n                  2                                ⁡                                  (                                                            n                      2                                        -                    1                                    )                                            ⁢              sin              ⁢                                                          ⁢              θcosθ                                                      (                                                      n                    2                                    -                                                            sin                      2                                        ⁢                    θ                                                  )                                            5                /                2                                              ⁢                      NA            3                                              (        2        )            wherein the refractive index (n) and thickness (d) of the recording medium denote a refractive index and a thickness of an optical medium that ranges from a light incidence surface to a recording surface, respectively.
Considering Equation 2, when a numerical aperture of an objective lens is increased to achieve high-density recording, the thickness of a recording medium should be reduced to secure a tolerance generated due to a tilt of the recording medium.
Hence, when the numerical aperture of the objective lens is increased to 0.85 to deal with a high-density recording medium, the thickness of the high-density recording medium should be about 0.1 mm. The high-density recording medium, which requires an objective lens with a high numerical aperture and has a thin thickness, is a Blu-ray disc (hereinafter, referred to as a BD). In the specification of a BD, a wavelength of a light source is 405 nm, a numerical aperture of an objective lens is 0.85, and a thickness of the BD is about 0.1 mm. The thickness of a recording medium denotes a distance between a surface of the recording medium on which light for recording and/or reproduction is incident and a recording surface of the recording medium.
Examples of a high-density recording medium include a BD and an advanced optical disc (AOD). An AOD uses a light source with a 405 nm light source and an objective lens with a 0.65 numerical aperture and has a thickness of 0.6 mm corresponding to the thickness of a DVD. BDs have higher recording densities than those of AODs.
Since a reflective ratio of a DVD-R, which is an example of an existing recording medium, significantly decreases depending on a wavelength of light, a light source with a 650 nm wavelength must be used.
Hence, to compatibly use both a high-density recording medium, (e.g., a BD), and a low-density recording medium, (e.g., a DVD), an optical recording and/or reproducing apparatus for high-density recording media generally uses an optical pickup having a separate optical system for high-density recording media and a separate optical system for low-density recording media, in consideration of both a spherical aberration generated due to a difference between thicknesses of recording media and numerical apertures required for recording media with different formats.
In other words, an optical pickup for high-density recording media generally has an optical system that uses both an objective lens for high-density recording media and an objective lens for low-density recording media. Also, considering the compatibility with DVD-R, the optical pickup for high-density recording media uses at least two light sources emitting beams with different wavelengths.
As described above, an optical pickup for high-density recording media should include at least two objective lenses to compatibly employ both high-density recording media and low-density recording media, for example, BDs and DVDs. In this case, the objective lenses maybe tilted toward each other due to an assembly causing performance degradation.
In other words, an objective lens for high-density recording media (i.e., BDs) and an objective lens for low-density recording media (i.e., DVDs) are generally mounted on a single lens holder such that the two objective lens are in focusing and tracking directions by a single actuator.
In such a system having two objective lenses, when a tilt between the two objective lenses occurs, an optical axis of one of the two objective lenses may be adjusted to be perpendicular to a recording medium using skew adjustment. However, the other objective lens is tilted with respect to the recording medium. Consequently, because features of an optical system, such as a light spot skew, should be adjusted as much as possible to an objective lens for BDs, features of an optical system for DVDs may be degraded.
Also, when an optical system for high-density recording media and an optical system for low-density recording media are separately installed in an optical system as described above, an optical system of the optical pickup is very complex, and a large number of optical components are required. Such a complicated optical system causes an increase in the material cost, a decrease in the reliability, and a degradation of the performance of the assembly and adjustment. The reliability decrease is due to deviation of the optical components during an operation at a high temperature. As the number of optical components increases, the number of points on which the optical components are attached to a base frame using an adhesive also increases. Hence, during a high-temperature operation, the probability that the optical components are deviated increases, and the degree of distortion of the entire optical system becomes severe. Further, as the number of optical components increases, an assembling error during assembly increases.